This invention relates to a method and apparatus for producing methane, and more particularly to such a means and apparatus which relies upon methanogenic bacteria to convert a carbon source to methane.
With the world facing a dwindling supply of sources of so-called fossil fuels, many have come to believe that energy in the future will be provided through the generation and use of hydrogen. There are many problems, however, associated with this approach. For one thing, each molecule of methane possesses much more energy when combusted than a molecule of hydrogen. Natural gas is essentially methane, and all our natural gas lines are geared to the transport of methane rather than hydrogen. Hydrogen is a far more explosive material than methane. While methane technology has been fairly well developed, fairly little has been perfected in connection with hydrogen technology. Hydrogen, therefore, should be converted to methane. The need, therefore, exists for a methane generation process which is practical, nonlabor intensive, sanitary, efficient, and, in essence, provides a renewable source of methane.
The production of methane using methanogenic bacteria is not new. However, serious drawbacks are associated with known techniques for producing methane in this manner. One process which has received considerable attention in recent years is what is referred to herein as a marine biomass process, wherein an organic material such as kelp, through the action of bacteria in a fermentation process, is first broken down into simpler organic components, with these simpler components than being further broken down to materials such as acetate which are converted through further reactions with bacteria to methane. Unfortunately, however, the use of kelp or similar materials tends to be rather expensive, which becomes readily apparent when problems associated with kelp growing, harvesting, fermenting, and the elimination of waste end products, are taken into consideration. Because of the slow and inefficient nature of the process, the approach, at this time, does not appear to offer a commercially practical manner of producing methane. Marine biomass conversion is discussed in some detail in "Proceedings of a Workshop on Environmental Impacts of Marine Biomass", prepared by Lawrence Berkeley Laboratory, University of California, for Gas Research Institute, 8600 West Bryn Maur Mayr Avenue, Chicago, Ill.; (Final Report; February-October, 1981).
Another biological process for methane production is disclosed in U.S. Pat. No. 1,940,944. The process of this patent transforms carbon monoxide and hydrogen present in illuminating gas to methane, the gas being introduced to a stoppered flask which has within it a bacteria deposit. Producing methane from gaseous carbon monoxide introduces the obvious problem of providing a proper source of the gaseous carbon monoxide, and providing reaction vessels of sufficient size to handle the large volumes of gas needed to produce commercial quantities of methane.
Anaerobic fermentation of organic wastes has also been studied which is related to the naturally occurring methane production which occurs, for instance, in sediment found in rivers, swamps and bays. Studies have indicated that methane production in an anaerobic fermentation of organic waste may largely be due by reason of metabolism of materials such as acetic and propionic acids, these materials showing increased concentrations in failing fermentations. Such studies have made it appear that hydrogen and acetate metabolism by methanogenic bacteria may be related energetically, and that the presence of an excess of hydrogen may exert an inhibitory effect on the decomposition of acetate. Reference is made to "Kinetics of Hydrogen Assimilation in Methane Fermentation" by Timothy G. Shea et al., August, 1968, distributed by National Technical Information Service, U.S. Department of Commerce, 5285 Port Royal Road, Springfield, Va. In this type of process, as in a marine biomass process, many competing complex reactions are occurring, thus introducing problems in controlling operating conditions whereby significant amounts of methane are produced in a fast and efficient manner.
Generally, an object of this invention is to provide a unique process for producing methane utilizing methanogenic bacteria which obviates or minimizes many of the problems associated with processes such as those described above.
More particularly, the invention includes amongst its objects producing methane through bacteria reactions utilizing as a carbon source a water-dissolved mineral carbonate. The carbonate equilibrates in water producing dissolved carbon dioxide and the solution is essentially free from carbon compounds subject to microbial methanogenic reaction in reactions which are competitive with metabolic reduction of carbon dioxide.
More particularly, the invention contemplates the utilization of mineral carbonates as the source of carbon dioxide which have limited solubility (such term as used herein, referring to solubility of less than about 0.1 grams per 100 milliliters of water at 15.degree. C.). A bed of such material when submerged in a volume of water functions continuously to supply to the water dissolved carbon dioxide available for conversion to methane. As carbon dioxide is used up by converting to methane, additional carbonate is dissolved. The undissolved mass of material in the bed, in addition, provides attachment sites for the methanogenic bacteria which are reacting to convert the carbon dioxide to methane.
This invention further contemplates a proces wherein methanogenic bacteria within a volume of water confined within a vessel continuously produce methane, the hydrogen needed for supplying the energy source for such conversion being introduced at least periodically (ordinarily continuously) by introducing a flow of gas into the vessel, and carbon dioxide being supplied continuously through dissolution of mineral carbonate.
Further contemplated in the invention is the continuous introduction of water to the vessel containing the methanogenic bacteria, and withdrawal of water from such vessel of water which is displaced by the introduced water. In this manner, the mineral ion content of the water is held relatively constant. The water displaced and containing mineral ions produced by the dissolving of the mineral carbonate reacts with carbon dioxide present in the air to reproduce the mineral carbonate which may then be reused in the production of methane.
These and other objects and advantages are attained by the invention, which is described hereinbelow in conjunction with the accompanying drawing, which illustrates, in simplified form, apparatus usable in practicing the invention.